A class-D amplifier generates a square wave of which the duty cycle is modulated to reproduce an audio signal applied to its input. Since the output is binary, the dissipation is generally much lower than that of a traditional linear amplifier. Also, since the low-frequency portion of the spectrum of the output signal of a class-D amplifier is essentially the wanted audio signal, and the high-frequency portion serves no purpose other than to make the waveform binary, the high-frequency portion of the output signal is typically removed with a passive low-pass filter made essentially with reactive components (e.g. an L-C filter) to maintain high efficiency. However, depending on the modulation scheme, a class-D amplifier can be “filter-less,” e.g. it would not require any filtering other than the intrinsic L-R filter of the load (e.g. a dynamic loudspeaker).
The filter-less operation is mostly evident during an idle state, when the input signal is very low or even zero. In that case, a “non-filter-less” amplifier, providing a high frequency signal to the load, will dissipate energy if the filter is not properly designed or operated. This does not happen with a filter-less amplifier where, for zero or small signal, only a small amount of high frequency energy is applied to the loudspeaker.
Many parameters contribute to the total power dissipation of a class-D amplifier. The ohmic conduction (aka “IR”) losses are usually the largest although commutation losses are also important. Commutation losses include the power used to drive the output transistors and the power that is used to make a transition at the output (e.g. current to charge and discharge the output parasitic capacitor).
A drawback of any switching amplifier is radio frequency (RF) emission. Since the binary transition is sharp, the output spectrum has a significant content in the radio-frequency range. The wires that connect the amplifier to the loudspeaker act as radiating antennas and can interfere with RF-sensitive equipment nearby. The RF emission depends on the slope of the binary signal, as well as on its amplitude. Since it is mandatory to be compliant with international standards and directives, if no other design solution is applied, RF emission must be reduced at the cost of external filtering components.
The maximum power delivered to the load depends on the amplitude of the binary signal which is typically equal to the supply voltage. To get more power from a certain supply, boost technique can be used, e.g. a Class-D amplifier supplied by a boosted voltage. In that case RF emission is higher because of the larger amplitude of the binary waveform and efficiency is lower due to the losses into the supplying DCDC converter.
A patent describing a class-D amplifier is U.S. Pat. No. 6,535,058 of Kirn entitled “Multi-Reference, High Accuracy Switching Amplifier.” Kirn does not, by way of non-limiting examples, address the issues of filter-less idle operation, amplifier efficiency or RF emission characteristics of his amplifier.
These and other limitations of the prior art will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.